1. Field of the Invention
The invention relates to a method of forming a thin insulating film containing fluorine (F) therein, and more particularly to a method of forming a fluorine-added insulating film having a low dielectric constant for electrically insulating wiring layers from each other in a semiconductor device having multiple wiring layers.
2. Description of the Related Art
With an increase in integration of LSI, it is quite important to form a multi-layer wiring structure with high reliability in fabrication of LSI. In a present multi-layer wiring structure, insulating films are sandwiched between wiring layers in order to avoid intersection and overlap of wiring layers. Such an interlayer insulating film is generally composed of material having a heat resistance and an electric insulation, such as silicon dioxide, silicon nitride and boronphosphosilicate glass. In addition, an interlayer insulating film is formed between wiring layers by chemical vapor deposition (CVD) for planarizing irregularities in wiring layers therewith.
However, with a design rule for LSI being smaller, there has been paused a problem of wiring delay caused by parasitic capacity between wiring layers, which problem is more serious than a problem of signal delay caused by individual devices such as a transistor constituting LSI. In other words, even if a device such as a transistor is fabricated in a smaller size for enhancing an operation speed thereof, an operation speed of LSI is not improved because of wiring delay. Under those circumstance, it is in urgent need to newly develop an interlayer insulating film having a smaller dielectric constant .epsilon.r than that of a silicon nitride film, which has a dielectric constant .epsilon.r equal to 7 or smaller, and a silicon dioxide film, which has a dielectric constant .epsilon.r equal to 3.9 or smaller, both of which are presently widely used as an insulating film in fabrication of LSI.
As a material having a low dielectric constant, there are widely used an insulating material composed mainly of silicon and an insulating material composed mainly of polymer. They have both merits and demerits. Namely, an insulating material composed mainly of silicon has an advantage that the existing LSI techniques can be applied thereto, but it is quite difficult to adequately lower a dielectric constant thereof. An insulating material composed mainly of polymer has an advantage that it is relatively easy to lower a dielectric constant thereof, but it does not match to the existing LSI processes, since it is a new material.
These days, with respect to an insulating film composed mainly of silicon, an attempt has been made to introduce fluorine into a silicon dioxide film to thereby lower a dielectric constant thereof, resulting in that there was obtained a fluorine-added silicon dioxide film having a dielectric constant of about 3.
With respect to an insulating film composed mainly of polymer, an amorphous carbon fluoride film which can be deposited by CVD has attracted an attention as a film having both a heat resistance and a low dielectric constant. An amorphous carbon fluoride film would have a dielectric constant of about 2 by introducing fluorine thereinto.
Thus, it is effective to introduce fluorine into the above-mentioned insulating films composed mainly of silicon and polymer in order to lower a dielectric constant thereof.
As mentioned above, an attempt has been made to use a fluorine-added insulating film such as a fluorine-added silicon oxide film (SiOF) and an amorphous carbon fluoride film, as an insulating film to be used for a multi-layer wiring structure. It is possible to control a dielectric constant of those fluorine-added insulating films by varying a content of fluorine therein. If a content of fluorine in the fluorine-added insulating films is increased, a dielectric constant of each of the films is lowered. It would be necessary to increase a content of fluorine in the films in order to lower a parasitic capacity among wiring layers.
When a fluorine-added silicon oxide film is formed by CVD, hydride of silicon such as silane (SiH.sub.4) and fluoride of silicon are employed as a process gas. A content of fluorine in the film is controlled by varying a supply ratio of silicon hydride and silicon fluoride. When an amorphous carbon fluoride film is formed by CVD, hydride of carbon such as methane (CH.sub.4) and fluoride of carbon such as carbon tetrafluoride (CF.sub.4) are employed as a process gas. A content of fluorine in the film is controlled by varying a supply ratio of carbon hydride and carbon fluoride. Herein, the term "hydride" means all compounds containing no hydrogen molecules and atoms in the specification.
However, the above-mentioned processes in which hydride is employed as a part of a process gas is accompanied with problems as follows.
A problem in a fluorine-added silicon oxide film is pointed out by H. Miyajima et al, "Formation Mechanism of F-added SiO.sub.2 Films using Plasma CVD", 1994 Dry Process Symposium, pp. 133-138. A fluorine-added silicon oxide film is deposited by CVD in which silane (SiH.sub.4), silicon tetrafluoride (CF.sub.4) and oxygen gases are used as process gases, with a volume ratio between silane and silicon tetrafluoride being varied to thereby control a content of fluorine in a resultant film. If fluorine were introduced into a film by tens of percents, a dielectric film of a resultant film would be lowered to about 3.5. However, the resultant film would have hygroscopic property at the same time, resulting in that moisture captured in the film reacts with fluorine to thereby cause hydrolysis of the resultant film. The hydrolysis in turn causes the film to peel off at an interface of the insulating film and a film making contact with the insulating film and composed of material different from that of the insulating film.
When an amorphous carbon fluoride film is fabricated, a dielectric constant thereof can be varied in the range of about 4 to about 2 by varying a mixture ratio of carbon fluoride gas and hydrocarbon gas. However, since hydrofluoric acid, which is a product of reaction, is also captured in a resultant film, hydrofluoric acid would be produced, if the resultant film is heated to about 100 degrees. It is possible to lower a dielectric constant of a resultant film by increasing a content of fluorine. However, if an aluminum film is formed on an amorphous carbon fluoride film highly containing fluorine, the aluminum film is often corrupted, and as a result, turned into black in color.
A problem of generation of hydrofluoric acid in a reaction product causes not only a problem that hydrofluoric acid is captured in a resultant film, but also a problem that hydrofluoric acid corrupts an underlying film composed of a material which is highly reactive with hydrofluoric acid, such as aluminum. As a result, a material of which an underlying film is composed is limited to specific materials which does not react with hydrofluoric acid, which narrows availability of a process for fabricating an insulating film.
As mentioned above, both of a fluorine-added silicon oxide film and an amorphous carbon fluoride film have problems considered to be caused by hydrofluoric acid which is produced as a reaction product when hydride is employed as a part of a process gas. In particular, such problems become remarkable, if a content of fluorine in a resultant film is increased for lowering a dielectric constant of the film.
In addition, there paused additional problems in association with an increase in a content of fluorine, as follows.
For instance, a problem associated with an amorphous carbon fluoride film is pointed out by K. Endo et al., "Fluorinated amorphous carbon thin film grown by plasma enhanced chemical vapor deposition for low dielectric constant interlayer dielectrics", Journal of Applied Physics, Vol. 78(2), 1995, pp. 1370-1372. If a content of fluorine in an amorphous carbon fluoride film is increased too much, there would be caused problems of degradation of a heat resistance, reduction in hardness, and reduction in adhesion with a film composed of a material different from that of the amorphous carbon fluoride film. In other words, an insulating property of an insulating film is degraded in annealing at a high temperature to be carried out after deposition of the film, a dielectric constant is increased, and an insulating film is peeled off in a polishing step for planarizing the film.
The same problems as mentioned above are paused also in a fluorine-added silicon oxide film. In addition, if a content of fluorine in the film were increased, the film would have an enhanced hygroscopic property, resulting in that the insulating film might be decomposed by moisture which the insulating film itself absorbs by virtue of the enhanced hygroscopic property.
In order to solve the above-mentioned problems, various attempts have been made. For instance, in order to enhance adhesion of an amorphous carbon fluoride film with other films, a content of fluorine at an interface at which the amorphous carbon fluoride film makes contact with another film or an underlying substrate is locally reduced. However, the above-mentioned problem caused by hydrofluoric acid, a product of reaction, cannot be completely solved by the method in which a content of fluorine in an insulating film is varied by varying a mixture ratio of a fluorine-family gas and a hydrogen-family gas. Hence, it is not possible to completely prevent hydrofluoric acid captured in an insulating film from degrading a dielectric strength and corroding metal or other insulating films with which the insulating film makes contact.
Japanese Unexamined Patent Publication No. 8-97199 has suggested a method of forming a fluorine-added insulating film. A fluorine-added insulating film is formed by plasma-enhanced CVD where an electric power having at least two radio-frequencies one of which is 30 MHz or greater. In accordance with the suggested method, the number of fluorine radicals and ions is increased, and those fluorine radicals and ions react with hydrogen and carbon existing in a plasma. A product produced by the reaction is discharged out of a plasma-enhanced CVD apparatus. As a result, hydrogen and carbon reach a substrate, and hence, a coefficient of moisture absorption of a resultant insulating film is prevented from increasing due to reaction with hydrogen and oxygen existing in atmosphere.
However, an insulating film formed by the above-mentioned method suggested by Japanese Unexamined Patent Publication No. 8-97199 is accompanied with the same problem as mentioned above. Namely, hydrofluoric acid still remains in an insulating film formed by the above-mentioned method, and hence, it is not possible to solve the problems caused by hydrofluoric acid.